Engine Misfire on 4

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Happened a couple of times. Get Engine Light on, code is misfire on 4, engines lumpy. Changed coil pack and plug, checked wiring, looks ok. If I switch off, Reset warning light and start again she’s fine for a couple of hundred miles. Any clues. Thank you.
 

mercedes13156

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I'm assuming you replaced both the plug and coilpack with new items and that you're running the 4 cylinder engine.

I'm assuming that the other 3 cylinders are running without misfires. I'm also assuming from that, that you have fuel and air in the correct proportions and you say the wiring "looks OK". If you have two out of the three right, then the other one has to be the problem. That leaves the electrics and if you've changed the plug and coil pack, then it should all work. If it's a fault prior to the coilpack installation, the whole engine will be affected. If it's just in one cylinder, it's restricted to .... the connection to the loom / ECU, the wiring from the loom / ECU to the coilpack, the coil pack itself, the HT wiring from the coilpack to the plug and finally the connection between the HT lead and the plug. Indeed, it could be a combination of problems from that list.

The best way to identify the culprit is to move the original plug to another cylinder and retest to see if the problem moved with it. If it didn't, then you move the original coil pack to another cylinder, (not the one you moved the plug to) and see if the problem migrates with it.

It would also help to know where on the rev range the misfire occurs.
 
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I'm assuming you replaced both the plug and coilpack with new items and that you're running the 4 cylinder engine.

I'm assuming that the other 3 cylinders are running without misfires. I'm also assuming from that, that you have fuel and air in the correct proportions and you say the wiring "looks OK". If you have two out of the three right, then the other one has to be the problem. That leaves the electrics and if you've changed the plug and coil pack, then it should all work. If it's a fault prior to the coilpack installation, the whole engine will be affected. If it's just in one cylinder, it's restricted to .... the connection to the loom / ECU, the wiring from the loom / ECU to the coilpack, the coil pack itself, the HT wiring from the coilpack to the plug and finally the connection between the HT lead and the plug. Indeed, it could be a combination of problems from that list.

The best way to identify the culprit is to move the original plug to another cylinder and retest to see if the problem moved with it. If it didn't, then you move the original coil pack to another cylinder, (not the one you moved the plug to) and see if the problem migrates with it.

It would also help to know where on the rev range the misfire occurs.

Thanks for coming back. Will try swapping say 4 for 3 and see what happens. Otherwise it’s just a question of tracing I suppose. When it happens engine seems to get worse. Rougher on idle and awful in traffic. Feels more like the cylinder just not working. Strange tho when I reset E light seems to be ok for some while. No actual misfire just lumpy.
 

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If the ECU detects misfires it will allow it for a little while then eventually turn off the injector to that cylinder to prevent fuel entering the catalytic converter and wrecking it.
 

mercedes13156

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If moving and testing the electrics don't find the issue, take the oil filler cap off at idle and see if it's blowing too hard. it might be a broken piston ring letting compression out into the crankcase, or even a sticky valve not opening or closing properly.
 

mercedes13156

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If the ECU detects misfires it will allow it for a little while then eventually turn off the injector to that cylinder to prevent fuel entering the catalytic converter and wrecking it.

If the engine's just "lumpy" at low revs. It might not be electrical. Possibly a sticky valve not emptying the exhaust gas properly, or not allowing a good fuel/air charge and igniting badly causing poor compression and/or poor readings across the O2 sensor? That would make it lumpy and probably register a misfire.
 

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If the engine's just "lumpy" at low revs. It might not be electrical. Possibly a sticky valve not emptying the exhaust gas properly, or not allowing a good fuel/air charge and igniting badly causing poor readings across the O2 sensor?
Could equally be a bad injector.
A bad reading at the O2 sensor will affect all cylinders.
 

mercedes13156

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Could equally be a bad injector.
A bad reading at the O2 sensor will affect all cylinders.

Very true! But you'd probably still register the misfire on the poorly performing cylinder which it has done. Food for thought.
 
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Thanks for all this. What's puzzling is when I reset and carry on everything is fine. It goes as it should for sometime before it happens again. Just driven back today over 100 miles no problems. It will probably come up again sometime?
 

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Very true! But you'd probably still register the misfire on the poorly performing cylinder which it has done. Food for thought.
But then it would give random cylinders each occurence.
From what I've seen an O2 sensor will give a "Random misfire" fault code.
 
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Ok swapped 3 coil pack and plug for 4. Checked all wiring and injectors - all seems ok. Removed oil cap when engine on idle - some air passing I assume this is normal. Doesn't seem excessive and as I say when I reset the ECL and switch back on all seems well. Wait and see what happens? The cars done 184k and I was hoping to make 200k. Weather is awful today.
 

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Ok swapped 3 coil pack and plug for 4. Checked all wiring and injectors - all seems ok. Removed oil cap when engine on idle - some air passing I assume this is normal. Doesn't seem excessive and as I say when I reset the ECL and switch back on all seems well. Wait and see what happens? The cars done 184k and I was hoping to make 200k.
Yes some air puffing from the filler is normal as long as it isn't excessive (i.e. no harder than you can breathe out your wide open mouth). Fingers crossed as to a result of some kind.
 
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Definitely a puff nothing else. I'll keep an eye on it. Many thanks for your help.
 
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Hi
What are the exact words of the error message.
I am sure it will make 200k.
Pretty sure it was P0304 Engine misfire on cylinder 4. From memory should have photo’d it.
 

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Pretty sure it was P0304 Engine misfire on cylinder 4. From memory should have photo’d it.
Hi
Thank you.
......" When the code P0304 is set in the Powertrain Computer, it means that the Misfire Monitor has detected more than a 2 percent variance in RPM between the firing of any two (or more) cylinders in the firing order."

Interesting. We need the experts to tell us more about the 'misfire monitor'.
 
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Hi
Thank you.
......" When the code P0304 is set in the Powertrain Computer, it means that the Misfire Monitor has detected more than a 2 percent variance in RPM between the firing of any two (or more) cylinders in the firing order."

Interesting. We need the experts to tell us more about the 'misfire monitor'.

Ok done it again. Error code P0304 Engine Misfire on 4. Also on pending codes P0204 injector circuit/open cylinder 4. (Might be my reader as in pending)
Reset took a bit longer before it sorted. Does feel like no fuel to no 4. Swapped 3 and 4 previously so assume not coil/plug. Any suggestions. Thanks.
 

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I am inclined to think it could be to do with the electrical connector to the injector on no.4 cylinder.
 

mattkh

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I am inclined to think it could be to do with the electrical connector to the injector on no.4 cylinder.
 
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Thinking it might be injectors. Will strip down and check everthing.
Thanks
 


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